Method and apparatus for on/off control of a power converter

ABSTRACT

A power converter is disclosed. An example power converter includes an energy transfer element coupled between a power converter input and a power converter output. A power switch is coupled to the energy transfer element and the power converter input. A feedback sampling circuit is coupled to receive a feedback signal representative of the power converter output to generate feedback signal samples during switching cycles. A switch conduction scheduling circuit is coupled to determine enabling and disabling of the power switch in future switching cycles in response to the feedback signal samples from a present switching cycle and one or more past switching cycles. A switch conduction control circuit is coupled to enable or disable conduction of the power switch during a switching cycle to control an amount of energy transferred from the power converter input to the power converter output.

RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.12/816,982, filed Jun. 16, 2010, now pending which is a continuation ofU.S. application Ser. No. 11/858,678, filed Sep. 20, 2007, entitled“Method and Apparatus for On/Off Control of a Power Converter,” nowissued as U.S. Pat. No. 7,764,520 B2, which claims benefit of U.S.Provisional Application No. 60/922,126, filed Apr. 6, 2007, entitled“Method And Apparatus For On/Off Control Of A Power Converter”. U.S.application Ser. Nos. 12/816,982 and 60/922,126 and U.S. Pat. No.7,764,520 B2 are hereby incorporated by reference.

BACKGROUND INFORMATION

1. Field of the Disclosure

The present invention relates generally to switching power convertersand, more specifically, the present invention relates to a technique toregulate an output of a switching power converter.

2. Background

Many electrical devices such as cell phones, personal digital assistants(PDA's), laptops, etc. are powered by a source of relatively low-voltageDC power. Because power is generally delivered through a wall outlet ashigh-voltage AC power, a device, typically referred to as a powerconverter, is required to transform the high-voltage AC power tolow-voltage DC power. The low-voltage DC power may be provided by thepower converter directly to the device or it may be used to charge arechargeable battery that, in turn, provides energy to the device, butwhich requires charging once stored energy is drained. Typically, thebattery is charged with a battery charger that includes a powerconverter that meets constant current and constant voltage requirementsrequired by the battery. In operation, a power converter may use acontroller to regulate output power delivered to an electrical device,such as a battery, that may be generally referred to as a load. Morespecifically, the controller may be coupled to a sensor that providesfeedback information of the output of the power converter in order toregulated power delivered to the load. The controller regulates power tothe load by controlling a power switch to turn on and off in response tothe feedback information from the sensor to transfer energy pulses tothe output from a source of input power such as a power line.

One particular type of power converter that may be used is a flybackpower converter. In a flyback power converter, an energy transferelement separates the input of the power converter from the output ofthe power converter. The energy transfer element typically providesisolation that prevents DC current from flowing between the input andthe output. In applications where isolation is not required, the inputand the output may share a common return terminal that allows DC currentto flow between the input and the output. A switching power converterproduces an output by periodically switching a power switch for one ormore switching cycles. An on/off control technique that regulates anoutput of a switching power converter is one that enables or disablesconduction of the power switch within a particular switching cycle. Whenenabled, the power switch may conduct current within a switching cycle.When disabled, there is no conduction for the entire duration of theswitching cycle.

A current limited on/off control is one that terminates the conductionof the power switch within an enabled switching cycle when the currentin the switch reaches a current limit. When conduction is enabled, thepower switch may conduct within the switching cycle until the current inthe power switch reaches a current limit or until the conduction timereaches a maximum value.

An apparatus to implement an on/off control technique in a powerconverter is generally referred to as an on/off controller. A typicalon/off controller decides whether to enable or to disable the conductionof the power switch for each switching cycle by sensing the output ofthe power converter at the end of the previous switching cycle. Thecontroller may also set the current limit of the switch to control theenergy delivered to the output in each switching cycle. By controllingthe energy delivered to the output in a given cycle, the output may beinfluenced, thereby having an effect on whether conduction of the powerswitch is enabled or disabled in subsequent switching cycles. On/Offcontrol has advantages over alternatives in many applications thatdemand special combinations of features such as simplicity, low cost,fast transient response, and low power consumption at no load.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiment and examples of the presentinvention are described with reference to the following figures, whereinlike reference numerals refer to like parts throughout the various viewsunless otherwise specified.

FIG. 1 shows generally one example of a switching power converter thatuses a flyback topology and employs a control technique in accordancewith the teachings of the present invention.

FIG. 2 shows several switching cycles of waveforms from an exampleswitching power converter operating in accordance with the teachings ofthe present invention.

FIG. 3 shows an example portion of an example power converter employinga control technique in accordance with the teachings of the presentinvention.

FIG. 4 shows an example flow diagram 400 that describes a method toregulate an output of a switching power converter in accordance with theteachings of the present invention.

FIG. 5 shows generally an example power converter that includes anexample integrated circuit that implements a control technique inaccordance with the teachings of the present invention.

FIG. 6 shows internal details of an example integrated circuit thatimplements a control technique in accordance with the teachings of thepresent invention.

FIG. 7A shows generally a state diagram of one example of the statemachine of an example integrated circuit that implements a controltechnique in accordance with the teachings of the present invention.

FIG. 7B shows example parameters that correspond to each state of theexample state diagram of FIG. 7A.

DETAILED DESCRIPTION

Methods and apparatuses for implementing an on/off control technique fora power converter in accordance with the present invention aredisclosed. In the following description, numerous specific details areset forth in order to provide a thorough understanding of the presentinvention. It will be apparent, however, to one having ordinary skill inthe art that the specific detail need not be employed to practice thepresent invention. In other instances, well-known materials or methodshave not been described in detail in order to avoid obscuring thepresent invention.

Reference throughout this specification to “one embodiment”, “anembodiment”, “one example” or “an example” means that a particularfeature, structure or characteristic described in connection with theembodiment or example is included in at least one embodiment of thepresent invention. Thus, appearances of the phrases “in one embodiment”,“in an embodiment”, “one example” or “an example” in various placesthroughout this specification are not necessarily all referring to thesame embodiment or example. Furthermore, the particular features,structures or characteristics may be combined in any suitablecombinations and/or subcombinations in one or more embodiments orexamples. In addition, it is appreciated that the figures providedherewith are for explanation purposes to persons ordinarily skilled inthe art and that the drawings are not necessarily drawn to scale.

As will be discussed, examples according to the teachings of the presentinvention include a power converter controller including on/off controlthat enables or disables conduction of a power switch for one or morefuture switching cycles, not just for the present switching cycle.“On/off” herein refers to whether the power switch is enabled toconduct. An “on” cycle is one wherein the power switch is enabled, andtherefore, may conduct during that cycle and an “off” cycle is onewherein the power switch is disabled, or prevented from conducting.Thus, “on/off” in the present disclosure does not refer to whether thepower switch is, in fact, conducting in a given cycle, only whether ornot conduction of the power switch is enabled. In one example, theon/off control senses the output of the power converter only during anenabled cycle of the power switch, and then determines the number offuture cycles of the power switch to be enabled or disabled in responseto the samples of a sensed feedback signal in the present and pastswitching cycles. Thus, the on/off control may schedule future switchingcycles to be enabled and disabled in response to a history of eventsthat may include past and present samples of a sensed feedback signal aswell as past and present values of a current limit of the power switch.An advantage of the example on/off control is that it does not need tosense the output for future disabled cycles. The reduction orelimination of the need to sense the output at each switching cyclereduces the number of components and improves sensing accuracy inaccordance with the teachings of the present invention. Thus, improvedregulation at lower cost and higher efficiency compared to knownsolutions is provided.

To illustrate, FIG. 1 shows one example of a regulated switching powerconverter 100, sometimes referred to as a power supply, in accordancewith the teachings of the present invention. In the particular exampleshown in FIG. 1, switching power converter 100 is a power converterhaving a flyback topology. It is appreciated, however, that there aremany other known topologies and configurations of switching powersupplies that could also employ on/off control in accordance with theteachings of the present invention, and that the flyback topology shownin FIG. 1 is provided for explanation purposes.

The power converter in FIG. 1 provides output power to a load 140 froman unregulated input voltage V_(IN) 105. The input voltage V_(IN) 105 iscoupled to an energy transfer element T1 125 and a power switch S1 120,hereinafter referred to as the switch S1 120. In the example of FIG. 1,the energy transfer element T1 125 is a transformer with a primarywinding 122 and a secondary winding 128. A “primary winding” may also bereferred to as an “input winding” and a “secondary winding” may also bereferred to as an “output winding.” A clamp circuit 110 is coupled tothe primary winding 122 of the energy transfer element T1 125 to limitthe maximum voltage on the switch S1 120. Switch S1 120 is closed,thereby allowing current to be conducted through the switch, and opened,thereby substantially terminating conduction through the switch, inresponse to a control block 185. Thus, a switch that is closed may bereferred to as being “on,” whereas a switch that is open may be referredto as being “off.” In one example, switch S1 120 is a transistor. In oneexample, control block 185 may be implemented as an integrated circuitor may be implemented with discrete electrical components or acombination of discrete and integrated circuits. During operation of thepower converter, the switching of switch S1 120 produces pulsatingcurrent in the rectifier D1 130 that is filtered by capacitor C1 135 toproduce a substantially constant output voltage V_(O) or output currentI_(O) at the load 140.

The output quantity to be regulated is U_(O) 145, which in general couldbe an output voltage V_(O), an output current I_(O), or a combination ofthe two. A sense circuit 150 is coupled to sense the output quantityU_(O) 145 as a sense signal U_(SENSE) 155.

As shown in the depicted example, a feedback circuit 160 is coupled tothe sense signal U_(SENSE) 155 to produce a feedback signal U_(FB) 165that is sampled by a feedback sampling circuit 170. A sampled feedbacksignal U_(FB)* 175 is an input to a control block 185. The sampledfeedback signal U_(FB)* 175 gives an indication of the magnitude of thepower converter output, such as for example whether the output is higheror lower than a reference value. Another input to control block 185 is acurrent sense signal 190 that senses a current ID 115 in the switch S1120. Any of the many known ways to measure a switched current, such asfor example a current transformer, or the voltage across a discreteresistor, or the voltage across a transistor when the transistor isconducting, may be used to measure the current ID 115. A sampling signalP_(S) 180 from control block 185 activates the feedback sampling circuit170 to sample the feedback signal U_(FB).

In operation, the control block 185 operates switch S1 120 tosubstantially regulate the output U_(O) 145 to its desired value.Control block 185 typically includes an oscillator that defines aswitching cycle of duration T. Regulation is accomplished by control ofone or more switching parameters that determine the amount of energytransferred from the input to the output of the power converter. In oneexample, the maximum value of current I_(D) 115 is controlled. In oneexample, the switch S1 120 is controlled to conduct for a portion of aswitching cycle or to have no conduction in a switching cycle.

FIG. 2 shows generally several switching cycles with waveforms of thecurrent I_(D) 115 and the sampling signal P_(S) 180 from the switchingpower converter example of FIG. 1. As shown in the described example, aswitching cycle where the current I_(D) 115 is greater than zero isreferred to as an “Enabled” cycle. Thus, the switch is conducting duringat least a portion of an Enabled cycle. A switching cycle where currentI_(D) 115 is not substantially greater than zero is referred to as a“Disabled” cycle. A “Disabled” cycle may also be referred to as a“Skipped” cycle. A Disabled cycle is one that has the conduction ofswitch S1 120 disabled. Thus, the switch is prevented from conducting,for example by being held off or prevented from being turned on, duringthe entirety of a Disabled cycle.

During each Enabled cycle, switch S1 120 conducts current ID 115 untilthe current I_(D) 115 reaches one of a plurality of current limit valuesranging from a lower value I_(LIM1) through a higher value I_(LIMX). Thecontrol block 185 sets the current limit in each Enabled cycle to avalue that achieves the desired behavior of the power converter. In theexample illustrated in FIG. 2, the current limit is set to I_(LIM2) inthe Enabled cycle T₀ 205, the current limit is set to I_(LIMX) in theEnabled cycle T₁ 210, and the current limit is set to I_(LIM1) in theEnabled cycle T_(N) 215.

In one example, the sampling signal P_(S) 180 activates the feedbacksampling circuit 170 to sample the feedback signal U_(FB) 165 in eachEnabled cycle after a delay time t_(DLY) 220 from the time that switchS1 120 terminates its conduction.

FIG. 3 shows an example portion 300 of the example power converter ofFIG. 1 in greater detail. In FIG. 3, the sense circuit 150 includes asense winding 305 on the transformer 125 to sense the output quantity145 that is a voltage V_(O) on the capacitor C1 135. Resistors 310 and315 produce the sense signal U_(SENSE) 155 that is received by feedbackcircuit 160. In one example, sense signal U_(SENSE) 155 is a voltage. Inanother example, sense signal U_(SENSE) is a current. In one example,feedback circuit 160 includes a voltage comparator 340 that compares thevoltage of sense signal U_(SENSE) 155 to a threshold voltage V_(TH) 350,with both voltages measured with respect to a common input return 320.The output of comparator 340 is the feedback signal U_(FB) 165. In theexample, feedback signal U_(FB) 165 is high when the sense signalU_(SENSE) 155 is greater than the threshold voltage V_(TH) 350, andfeedback signal U_(FB) 165 is low when the sense signal U_(SENSE) 155 isless than the threshold voltage V_(TH) 350.

In the illustrated example, the feedback sampling circuit 170 is a Dflip-flop 330 that receives the feedback signal U_(FB) 165 to producethe sampled feedback signal U_(FB)* 175 when the sampling signal P_(S)180 from control block 185 clocks the D flip-flop 330. Thus, the sampledfeedback signal U_(FB)* indicates the magnitude of the output inaccordance with the teachings of the present invention.

In the example of FIG. 3, switch S1 120 is a Metal Oxide SemiconductorField Effect Transistor (MOSFET). In the example of FIG. 3, the brokenline 360 may represent the boundary of an integrated circuit thatincludes switch S1 120, control block 185, current sense signal 190,feedback sampling circuit 170, and feedback circuit 160 in a singlemonolithic device. In other examples, one or more of these functionalblocks may be implemented with discrete circuit components, monolithicintegrated circuits, hybrid integrated circuits, or various combinationsthereof.

FIG. 4 shows an example flow diagram 400 that describes a method toregulate an output of a switching power converter with on/off control inaccordance with the teachings of the present invention. After startingin block 405, all stored information from previous switching cyclesincluded in the control history is reset to a set of initial conditionsin block 410. The control history is a memory of one or more past eventsthat may include current limits, values of the sampled feedback signal,the number of enabled and disabled switching cycles, and any otherinformation useful to determine future enabled and disabled switchingcycles to meet the regulation requirements of a given application. Inone example, the control history is contained in the state of a digitalstate machine. The control proceeds to block 415 where conditions andinformation stored from previous switching cycles are evaluated. Next,the parameters for subsequent switching cycles are set in block 420.These parameters may include the current limit and the decision toschedule the switch to be enabled or disabled for one or more futureswitching cycles. Block 425 marks the end of the present switchingcycle. A new switching cycle starts in block 430.

Block 435 branches the control to either block 440 or block 455. If theparameters set in block 420 indicate that the switch is scheduled to beenabled at the next switching cycle, then the control branches to block440. If the parameters set in block 420 indicate that the switch isscheduled to be disabled for the next switching cycle, then the controlbranches to block 455.

When the switch is enabled, block 440 turns on the switch, allowing itto conduct, until the current in the switch reaches the current limit.When the current in the switch reaches the current limit, conduction isterminated, for example by turning the switch off. Then the controlmoves to block 445 to wait for a delay time before moving to block 450where it samples the feedback signal U_(FB). Control continues afterfeedback sampling to block 455 where the control history is updated. Thecontrol history is then evaluated in block 415 to continue the operationthrough block 420 before reaching the end of the switching cycle inblock 425 and starting a new switching cycle in block 430.

FIG. 5 shows generally one example of a power converter 500 that hasswitching, feedback and control functions included in an integratedcircuit 505 in accordance with the teachings of the present invention.In one example, the input voltage V_(IN) 105 is between 120 volts and375 volts. In one example, the output voltage V_(O) 145 is approximately5 volts. As shown in the illustrated example, clamp circuit 110 of thepower converter 500 includes a capacitor 510, a resistor 515, and arectifier 520. Integrated circuit 500 includes a drain terminal 525coupled to one end of the primary winding 122, a source terminal 540coupled to the common input return 320, a bypass terminal 530 coupled toa capacitor 545, and a feedback terminal 535 coupled to receive thesense signal U_(SENSE) 155 from the sense circuit 150.

FIG. 6 shows internal details of example integrated circuit 505 todescribe its operation. In the example, a power MOSFET 638 switchescurrent between the drain terminal 525 and the source terminal 540 inresponse to a switching signal 636 from an AND gate 634. An optionalvoltage regulator 602 coupled to the drain terminal 525 regulates thevoltage on the bypass terminal 530 to 5.8 volts when the MOSFET 638 isoff. Bypass terminal 530 provides an internal supply voltage 610 topower the internal circuits of integrated circuit 505. An externalcapacitor 545 shown in FIG. 5 coupled to the bypass terminal 530 storesenergy to power the internal circuits while the MOSFET 638 is on.

An optional hysteretic comparator 606 monitors the internal supplyvoltage 610 against a reference voltage 608. The hysteresis ofcomparator 606 causes reference voltage 608 to be either 4.8 volts or5.8 volts. The output 604 of hysteretic comparator 606 goes low to turnoff the MOSFET 638 through AND gate 634 when the internal supply voltage610 falls below 4.8 volts. When the internal supply voltage 610 rises to5.8 volts, the output 604 of hysteretic comparator 606 goes high, andthe reference voltage 608 drops to 4.8 volts.

As shown in the depicted example, the output of hysteretic comparator606 provides a reset signal 604 to a state machine 612. It isappreciated that in other examples, other internal or external circuitsmay provide the reset signal 604 as necessary for a given application. Atransition from low to high of reset signal 604 initializes the statemachine 612. In one example, the state machine 612 uses ordinary digitalcircuits such as for example logic gates, flip-flops, latches, counters,and the like, to produce outputs and schedule future switching cyclesthat are enabled or disabled in response to past and present digitalinput samples from the feedback sampling circuitry to provide on/offcontrol of the power converter in accordance with the teachings of thepresent invention.

The AND gate 634 receives the reset signal 604, a thermal shutdownsignal 630 from an optional thermal shutdown circuit 640, and an outputsignal 628 from the state machine 612. The switching signal 636 from theoutput of AND gate 634 goes low to turn off the MOSFET 638 whenevereither the reset signal 604 or the thermal shutdown signal 630 goes low.The thermal shutdown circuit 640 causes the thermal shutdown signal 630to go low when the junction temperature of the integrated circuitexceeds a threshold temperature value. Thus, the thermal shutdowncircuit 640 causes the MOSFET 638 to turn off when the junctiontemperature of the integrated circuit is too high.

As shown in the depicted example, an oscillator 624 provides a digitaltiming signal D_(TIMING) 626 to the state machine 612. In the depictedexample, digital timing signal 626 determines both the start of eachswitching cycle and the maximum time the MOSFET 638 may be turned on ineach switching cycle. As will be understood, oscillator 624 couldoptionally provide separate signals to control the initiation of a cycleand the maximum on time. In one example, the duration of each switchingcycle is approximately 15 microseconds.

Comparator 618 compares a sense signal at the feedback terminal 535 to athreshold V_(TH) 620. D flip-flop 616 samples the output of comparator618 to produce a sampled feedback signal U_(FB)* 614 at a sampling timedetermined by a sampling signal 622. The sampling time is the time theMOSFET 638 turns off, delayed by a sample delay time. Sampling signalgenerator 650 delays the signal 636 at the gate of MOSFET 638 by thesample delay time, and generates the sampling signal 622 at the delayedfalling edge of the signal 636. In one example, the sample delay time is2.5 microseconds.

Current limit comparator 644 compares a signal 642, which isproportional to the current in the MOSFET 638, with a current limitreference 632 from the state machine 612. In one example, the currentlimit reference 632 is based on a current state of the state machine612. The output of the current limit comparator 644 goes high toindicate when the current in MOSFET 638 reaches the current limitreference value 632. Switching signal 636 is delayed by leading edgeblanking circuit 654 before being applied to the input 652 of AND gate646 to prevent the current limit input 648 to the state machine 612 fromindicating a false current limit condition when MOSFET 638 momentarilydischarges stray capacitance as it turns on. In response to currentlimit input 648 indicating that the current in MOSFET 638 has reachedthe current limit reference value 632, state machine 612 instructs theswitch to terminate its conduction through operation of signal 628 andAND gate 634.

As shown in the illustrated example, the state machine 612 includes twooutputs. A first output 628 is gated by AND gate 634 to turn the MOSFET638 on and off. A second output 632 sets the reference of the currentlimit comparator 644. Thus, the state machine 612 enables or disablesthe MOSFET 638 from conducting in each switching cycle, controls thetermination of conduction during an enabled cycle, and also sets thecurrent at which the MOSFET 638 turns off after it is enabled inaccordance with the teachings of the present invention.

FIG. 7A shows an example state diagram 700 that describes states S1 705through S14 745 and the example conditions to transition between thestates S1 705 through S14 745 of one example of the state machine 612 inthe integrated circuit of FIG. 6 in accordance with the teachings of thepresent invention. FIG. 7B shows examples of parameters that correspondto each state of the state diagram 700. In the examples shown in FIG.7B, the current limit L_(LIMIT) of the switch S1 120 for each state isgiven as a percentage of the current limit value I_(LIMX) in accordancewith the teachings of the present invention. FIG. 7B also shows for eachstate example numbers of future switching cycles scheduled to be enabledor disabled after the state machine receives a low or a high levelsampled feedback signal U_(FB)* 614 in accordance with the teachings ofthe present invention.

At the start of operation, reset signal 604 sets the state machine 612to the S1 state 705. The state machine 612 transitions states when itreceives a number of consecutive high or low values of the sampledfeedback signal U_(FB)* 614. In the example of FIG. 7A, either three orseven consecutive high or low values of the sampled feedback signalU_(FB)* 614 are required to transition states as indicated in the statediagram 700.

In all states of this example, when the sampled feedback signal U_(FB)*614 is sampled low in a given switching cycle, the next switching cycleis an Enabled cycle. When the sampled feedback signal U_(FB)* is sampledhigh in a switching cycle, a predetermined number of subsequentswitching cycles are Disabled cycles. At least one Enabled cycleimmediately follows the predetermined number of Disabled cycles. It isappreciated that the low and high logical values can be easily reversedwith appropriate logical inversions in the circuit. The number ofsubsequent switching cycles that are Disabled cycles depends on eachparticular state. In the example illustrated in FIG. 7A and FIG. 7B, thenumber of Disabled cycles generally increases in the higher numberedstates. However, since there are four different current limit levels inthe example, 70%, 80%, 90%, and 100% of current limit I_(LIMX), thenumber of Disabled cycles decreases when transitioning from state S2 710to S3 715, and from S4 720 to S5 725, and from S6 730 to S7 735, inother words when transitioning to a state with a lower current limit.Starting at state S7 735, when the current limit is at its lowestsetting, the number of Disabled cycles is increased in binary powers,doubling from one Disabled cycle to two Disabled cycles in S8 740 andcontinuing to 128 Disabled cycles in the highest state S14 745. It isappreciated that the increase of Disabled cycles in binary powers is amatter of convenience in this example and other examples may usedifferent schedules for future Disabled cycles.

When the power converter 500 has its maximum load, the state machine 612will be in state S1 705 where the current limit is highest and thenumber of Disabled cycles is lowest (only one Disabled cycle perfeedback sample). When the power converter 500 has its minimum load, thestate machine 612 will be in the highest state S14 745. In theillustrated example, state S14 745 has the lowest current limit and thehighest number of Disabled cycles per high feedback sample.

To transition from one state to another state, the example state machine612 receives two or more consecutive high or low values of the sampledfeedback signal U_(FB)* 614. For example, three consecutive high valuesof U_(FB)* 614 (i.e. no low value in the present and past two switchingcycles) will cause the state machine 612 to transition from state S1 705to state S2 710, whereas three consecutive low values of U_(FB)* 615(i.e. no high value in the present and past two switching cycles) willcause the state machine 612 to transition from state S2 710 to state S1705. In most cases of the example illustrated in FIG. 7A and FIG. 7B,three consecutive high or low values are needed to transition betweenstates. Three exceptions are from state S3 715 to state S2 710, fromstate S5 725 to state S4 720, and from state S7 735 to state S6 730. Inthese cases, the number of consecutive low cycles is increased fromthree to seven to avoid excessive changes between states that havedifferent current limits. Excessive changes between such states canproduce patterns of switching cycles that degrade the performance of thepower converter through reduced efficiency, increased ripple voltage atthe output, and/or audio noise.

The above description of illustrated examples of the present invention,including what is described in the Abstract, are not intended to beexhaustive or to be limitation to the precise forms disclosed. Whilespecific embodiments of, and examples for, the invention are describedherein for illustrative purposes, various equivalent modifications arepossible without departing from the broader spirit and scope of thepresent invention. Indeed, it is appreciated that the specific voltages,currents, frequencies, power range values, times, etc., are provided forexplanation purposes and that other values may also be employed in otherembodiments and examples in accordance with the teachings of the presentinvention.

These modifications can be made to examples of the invention in light ofthe above detailed description. The terms used in the following claimsshould not be construed to limit the invention to the specificembodiments disclosed in the specification and the claims. Rather, thescope is to be determined entirely by the following claims, which are tobe construed in accordance with established doctrines of claiminterpretation. The present specification and figures are accordingly tobe regarded as illustrative rather than restrictive.

What is claimed is:
 1. A power converter, comprising: an energy transferelement coupled between a power converter input and a power converteroutput; a power switch coupled to the energy transfer element and thepower converter input; a feedback sampling circuit coupled to receive afeedback signal representative of the power converter output to generatefeedback signal samples during switching cycles; a switch conductionscheduling circuit coupled to determine enabling and disabling of thepower switch in a plurality of future switching cycles in response tothe feedback signal samples for each present switching cycle and one ormore past switching cycles; and a switch conduction control circuitcoupled to enable or disable conduction of the power switch during aswitching cycle to control an amount of energy transferred from thepower converter input to the power converter output.
 2. The powerconverter of claim 1 wherein the switch conduction scheduling circuitcomprises a state machine coupled to determine switching of the powerswitch for future switching cycles in response to the feedback signalsamples for each present switching cycle and the one or more pastswitching cycles.
 3. The power converter of claim 1 further comprising acurrent sense circuit coupled to the power switch to sense a currentthrough the power switch.
 4. The power converter of claim 3 wherein thecurrent sense circuit is coupled to terminate conduction of power switchin response to the current through the power switch.
 5. The powerconverter of claim 4 wherein the switch conduction scheduling circuit iscoupled to adjust a current limit of the power switch in response to thefeedback signal samples for each present switching cycle and one or morepast switching cycles.
 6. The power converter of claim 1 wherein thefeedback sampling circuit includes a sampling signal generator coupledto generate a delay time from an end of a conduction of the power switchbefore the feedback signal samples are sampled.